Differential-pressure-type flow measuring devices, employing a flow restricting orifice and pressure taps upstream and downstream therefrom connected to pressure sensing means, are generally used for measuring the flows of many types of fluids. These devices, however, cannot be effectively used to measure the flow rates of gaseous streams having significant quantities of entrained particulates therein. The orifice and open pressure taps of such a device quickly become clogged with the particulate matter, which causes erroneous or erratic pressure readings and may necessitate ceasing operations for a time sufficient to permit the removal and cleaning of these components. Several existing devices for measuring the flow of a gas containing entrained particulates rely on a cleaning system which operates in situ to blast or flush deposits from the surfaces of the orifice plate and open pressure taps. These devices, of course, cause erroneous readings during the cleaning cycles, and additionally allow for the steadily deteriorating accuracy of flow readings between cleaning cycles as the particulates slowly build over time.
U.S. Pat. No. 4,671,109 discloses a flow measuring device having pressure sensing elements which are inserted perpendicularly into the flow stream through gate valves. As the flow readings deteriorate, the pressure sensing element is withdrawn from the flow stream to a point just past the valve gate, the valve is closed, and finally the pressure sensing element is completely withdrawn and cleaned before being reinserted. This cleaning activity, of course, results in a period of time during which flow cannot be measured.
U.S. Pat. No. 4,498,347 discloses a flow measuring device utilizing an internal Pitot tube which, as it becomes clogged, is mechanically rotated within the flow stream so as to point generally downstream. Thereafter a blast of purge gas is forced backward through the Pitot tube, to remove adhered solids. Again, it is impossible to obtain accurate pressure readings during the purging cycles.
U.S. Pat. No. 4,651,572 discloses a flow measuring venturi arrangement, wherein the venturi orifice and flow conduit pressure tap are lined with a barrier layer of a porous, wear-resistant material. A rinsing gas may be either constantly or intermittently directed backward through the barrier layer into the flow stream, to keep the venturi orifice and pressure tap free from accumulations of particulates. This method, however, causes contamination of the flow stream by the rinsing gas. Furthermore, particulate fines will, over time, cause the barrier layer to slowly clog, resulting in inaccurate flow readings and the necessity of more severe rinsing.
U.S. Pat. No. 4,572,007 discloses a device and method for repelling particulates from a gas permeable surface using thermophoresis. A particulate-free gas sample may be drawn from the clean side of the hot permeable surface.
Finally, a publication entitled "Thin Film by Conveyorized Atmospheric CVD". by N. M. Gralenski, presented at the ISHM-Internepeon Technical Seminar in Tokyo. Japan, on Jan. 18, 1983, discloses on page 6 a self-cleaning orifice, including two counter-rotating cylinders and associated scrapers, in conjunction with open pressure taps which lead to a conventional pressure measuring device. The counter rotation of the cylinders allows the accumulated particulate material to be removed by the scrapers, thereby resulting in a constant orifice size at steady state operation. However, the disclosed open pressure taps allow the build-up over time of particulates therein, causing a steady deterioration of the accuracy of the flow readings. These deposits may be removed by disassembling the pressure taps, during which time flow readings are not available.
It must be noted that the prior art referred to hereinabove has been collected and reviewed only in light of the present invention as a guide. It is not to be inferred that such diverse art would otherwise be assembled absent the motivation provided by the present invention.
It would be desirable to construct an accurate differential-pressure-type continuous flow measuring device, suitable for measuring the flow of a gaseous stream containing entrained particulates, which is simple to operate and reliable. Such a flow measuring device would not require disassembly and cleaning, would not contaminate the flow stream with a purge gas, and would not give steadily deteriorating flow data due to the accumulation of particulates at the orifice and/or pressure taps.